1. Field of the Invention
The present invention relates to a voltage controlled variable capacitor with a capacitance that varies depending on a control voltage, and to a voltage controlled oscillator using the same.
2. Description of the Related Art
As voltage controlled variable capacitors that can be used on a semiconductor integrated circuit, there are known two types of capacitance elements, varactor diodes and MOS capacitance elements. The varactor diodes which can be fabricated on a semiconductor integrated circuit have a small variable ratio of capacitance values (a ratio between a minimum capacitance value and a maximum capacitance value) as illustrated in FIG. 6, and have a problem that a large frequency variable width cannot be obtained when being used in a voltage controlled oscillator circuit, or the like.
A voltage controlled oscillator such as VCXO (Voltage Controlled Crystal Oscillator) has a function to control its oscillation frequency by varying a capacitance value incorporated in an oscillator circuit with a control voltage applied from an outside unit. As this variable capacitance element, a varactor diode is generally used. However, the varactor diode which can be fabricated on an integrated circuit has a ratio between a minimum capacitance value and a maximum capacitance value as small as about double. This is due to that a PN junction with a steep gradient of concentration cannot be achieved by a process for forming the integrated circuit.
On the other hand, as voltage controlled variable capacitors with a wide capacitance variable width that can be fabricated on an integrated circuit, MOS capacitors are known. A MOS voltage controlled variable capacitor can achieve a variable width of about double the variable width of the varactor diode (PN-junction diode). However, the MOS voltage controlled variable capacitor changes in capacitance value steeply at a voltage (approximately 0.5 V) in the vicinity of a threshold (Vt) of a MOS transistor, and thus its capacitance value is liable to be modulated by noise components contained in the control voltage. Consequently, phase noise in the voltage controlled oscillator is deteriorated, and moreover there is a drawback that the linearity of a relation between the control voltage and the oscillation frequency is poor.
Accordingly, in FIG. 10 of Japanese Patent Application Laid-open No. 2005-64691, there is proposed a variable capacitor in which capacitors are connected to gate sides of MOS capacitance elements formed of MOS transistors each having a short-circuited source and drain, series circuits which are each formed thus of a MOS capacitance element and a capacitor are connected in parallel, and a control voltage is applied to the short-circuited part of the source and the drain and bias voltages are applied between the MOS capacitance elements and the capacitors. In this variable capacitor, the bias voltages differ among the series circuits from each other. Accordingly, when the control voltage is increased sequentially, the voltage between the gate and the source (drain) of each MOS capacitance element exceeds a threshold sequentially, and the capacitance value of the MOS capacitance element decreases. Therefore, the variable ratio of capacitances is large, but there is a drawback that a Q value of the variable capacitor is low because a parasitic resistance formed of an inversion layer is inserted in series in the capacitor.
It is an object of the present invention to provide a voltage variable capacitor which has a large variable ratio of capacitance values (ratio between a minimum capacitance value and a maximum capacitance value), is capable of achieving a highly linear relation between a control voltage and an oscillation frequency when the capacitor forms a voltage controlled oscillator for example, and has a high Q value. It is another object of the present invention to provide a voltage controlled oscillator which achieves a highly linear relation between a control voltage and an oscillation frequency and has a large adjustable width of an oscillation frequency.
A voltage controlled variable capacitor according to the present invention includes:
(1) using a plurality of MOS capacitance elements each including an N-type well layer formed in a silicon layer, a gate electrode formed on the well layer via a gate oxide film, and a contact layer formed of an N+ layer containing a higher concentration of N-type impurities than that of the well layer and formed at a position separated from the gate electrode in a surface direction in the well layer;
(2) electrically connecting respective contact layers of the plurality of MOS capacitance elements in common;
(3) providing a bias voltage supply unit for supplying bias voltages different from each other to gate electrodes of the respective MOS capacitance elements;
(4) providing a plurality of non-variable capacitance elements having one ends connected respectively to the gate electrodes of the MOS capacitance elements, and the other ends connected in common; and
(5) setting a difference between bias voltages of adjacent voltages smaller than a threshold voltage Vt, where Vt is a threshold voltage at a time when a source and a drain are formed in the well layer of one of the MOS capacitance elements to make a MOS transistor,
in which a control voltage is supplied to the contact layers to thereby control a capacitance value between a common connection point of the plurality of MOS capacitance elements and a common connection point of the plurality of non-variable capacitance elements.
A capacitance portion of each of the non-variable capacitance elements is provided to overlap with the gate electrode for example.
A voltage controlled variable capacitor of another invention includes:
(1) using a plurality of MOS capacitance elements each including a P-type well layer formed in a silicon layer, a gate electrode formed on the well layer via a gate oxide film, and a contact layer formed of a P+ layer containing a higher concentration of P-type impurities than that of the well layer and formed at a position separated from the gate electrode in a surface direction in the well layer;
(2) electrically connecting respective gate electrodes of the plurality of MOS capacitance elements in common;
(3) providing a bias voltage supply unit for supplying bias voltages different from each other to contact layers of the respective MOS capacitance elements;
(4) providing a plurality of non-variable capacitance elements having one ends connected respectively to the contact layers of the MOS capacitance elements, and the other ends connected in common; and
(5) setting a difference between bias voltages of adjacent voltages smaller than a threshold voltage Vt, where Vt is a threshold voltage at a time when a source and a drain are formed in the well layer of one of the MOS capacitance elements to make a MOS transistor,
in which a control voltage is supplied to the gate electrodes to thereby control a capacitance value between a common connection point of the plurality of MOS capacitance elements and a common connection point of the plurality of non-variable capacitance elements.
A voltage controlled variable capacitor according to still another invention includes:
(1) using a plurality of MOS capacitance elements each including an N-type well layer formed in a silicon layer, a gate electrode formed on the well layer via a gate oxide film, and a contact layer formed of an N+ layer containing a higher concentration of N-type impurities than that of the well layer and formed at a position separated from the gate electrode in a surface direction in the well layer;
(2) electrically connecting respective contact layers of the plurality of MOS capacitance elements in common;
(3) providing a bias voltage supply unit for supplying bias voltages different from each other to gate electrodes of the respective MOS capacitance elements;
(4) setting a difference between bias voltages of adjacent voltages smaller than a threshold voltage Vt, where Vt is a threshold voltage at a time when a source and a drain are formed in the well layer of one of the MOS capacitance elements to make a MOS transistor,
in which a control voltage is supplied to the contact layers to thereby control a capacitance value between a common connection point of the plurality of MOS capacitance elements and gate electrode sides of the MOS capacitance elements.
A voltage controlled variable capacitor according to yet another invention includes:
(1) using a plurality of MOS capacitance elements each including a P-type well layer formed in a silicon layer, a gate electrode formed on the well layer via a gate oxide film, and a contact layer formed of a P+ layer containing a higher concentration of P-type impurities than that of the well layer and formed at a position separated from the gate electrode in a surface direction in the well layer;
(2) electrically connecting respective gate electrodes of the plurality of MOS capacitance elements in common;
(3) providing a bias voltage supply unit for supplying bias voltages different from each other to contact layers of the respective MOS capacitance elements;
(4) setting a difference between bias voltages of adjacent voltages smaller than a threshold voltage Vt, where Vt is a threshold voltage at a time when a source and a drain are formed in the well layer of one of the MOS capacitance elements to make a MOS transistor,
in which a control voltage is supplied to the gate electrodes to thereby control a capacitance value between a common connection point of the plurality of MOS capacitance elements and contact layer sides of the MOS capacitance elements.
According to another invention, in a voltage controlled oscillator using a voltage controlled variable capacitor, the voltage controlled variable capacitor according to the present invention is used.
According to the present invention, electrodes on one end sides of the plurality of MOS capacitance elements are connected in common and the control voltage is applied to the common connection point thereof. A bias voltage is applied to electrodes on the other end sides of the MOS capacitance elements via a bias resistor, and there is provided a plurality of non-variable capacitance elements having one ends connected respectively to the gate electrodes of the MOS capacitance elements without intervention of the bias resistor, and the other ends connected in common.
In another invention, electrodes on one end sides of the plurality of MOS capacitance elements are connected in common and the control voltage is applied to the common connection point thereof, and the bias voltage is applied to electrodes on the other sides of the MOS capacitance elements.
In any one of the inventions, voltages of adjacent bias voltages are set smaller than the threshold voltage. Thus, the capacitance values of the MOS capacitance elements decrease sequentially when the control voltage is increased. Therefore, it is possible to obtain a voltage-controlled variable capacitor which can be formed on an integrated circuit, has a large variable ratio of capacitances, and can achieve a highly linear relation between the control voltage and an oscillation frequency when a VCO is formed. Further, a voltage controlled variable capacitor with a high Q value can be obtained.